X-ray fluorescence (XRF) is a common technique used to identify the elemental composition of a sample. XRF is used in a variety of fields ranging from mining, to artwork analysis, to crime scene investigations, to planetary exploration. In traditional XRF, Energy Dispersive (ED) detectors are used to identify characteristic spectral line emission excited in a sample by an applied radiation source.
XRF uses the energy-dispersed identification of characteristic x-ray emission lines from elements that are excited by applied x-rays or high-energy particles; this applied radiation typically has a fixed spectrum that spans energies high enough to excite emission from a broad range of characteristic lines. An ED detector then uses its energy-resolving capability to identify the lines and thus the constituent elements. ED detectors with adequate energy resolution are typically small (<1 cm across) and must be in close proximity to the excited sample in order to preserve sensitivity. The characteristic photons disperse in random directions from the excited atom; to capture the characteristic photons, the small ED detector must be placed close to a sample to capture a large solid-angle, and more photons. Typical distances are comparable to the ED detector size, in the range of millimeters, or nearly in contact.
ED detectors are limited to small sizes for a number of reasons. Typically, ED detectors are silicon devices that are cooled to reduce thermal noise and increase energy resolution. Power requirements to cool large detectors can become complex and costly. Also, the fabrication of large, high quality, energy resolving detectors can be expensive due to the high purity of material required. Thus, larger ED detectors are costly and practically limited by power and availability.
NASA has identified resource identification and utilization as a means of supporting human exploration of the Moon, asteroids, Mars, and other Solar System bodies. Mars rover missions carry XRF units that use radioactive alpha-particle sources to stimulate XRF from samples encountered along their traverses. Commercial handheld units for use on Earth also exist. Both require near-contact for measurements for the reasons outlined above. For fine-scale mapping of extraterrestrial mineral resources, for example, astronauts would have to stop and exit their vehicles every few feet or so to make a contact-XRF measurement to map elemental composition, and resource extraction would go very slowly. Such exploration could benefit from a new XRF detection technique that does not require close proximity to a sample in order to determine its composition.